Objective: The objective of this serum bank is to provide a mechanism for prospectively evaluating relations of serum concentrations of various analytes such as hormones, antioxidant micronutrients, vitamin D, and environmental pollutants with breast cancer risk. Background: The serum bank was created as part of NCI's Biological Markers Project. Between 1977 and 1987, a total of 7,355 women free of breast cancer donated blood to the bank, and by the end of followup in 1989, 146 of these women had been diagnosed with breast cancer. Questionnaires that ascertained information on age, height, weight, reproductive history, medical history, medications (including exogenous estrogens), and family history of breast cancer were completed at each blood collection. Serum has been stored at -70 degree C since it was collected. Methods: The serum bank has been used for nested case-control studies of breast cancer in relation to sex hormones, antioxidant micronutrients, pesticides, and PCBs. Progress: We reported strong positive associations of bioavailable estradiol and testosterone with postmenopausal breast cancer in these women; those in the highest quartiles for these hormones were at 5- and 6-fold excess risks, respectively. Laboratory assays of antioxidants, pesticides, and PCB's have been completed and data analysis is underway. Additional sex hormones, IGF-1, c-peptide, and vitamin D currently are being measured, and estrogen, progesterone, and androgen receptor content of tumors is being determined.